The present invention relates to heat pumps, and particularly to the special design of a liquefier for a heat pump.
WO 2007/118482 discloses a heat pump with an evaporator for evaporating water as the working liquid to produce working vapor. The heat pump further includes a compressor coupled to the evaporator to compress the working vapor. Here, the compressor is formed as a flow machine, wherein the flow machine comprises a radial wheel accepting uncompressed working vapor at its front side and expelling same by means of correspondingly formed blades at its side. By way of the suction, the working vapor is compressed so that compressed working vapor is expelled on the side of the radial wheel. This compressed working vapor is supplied to a liquefier. In the liquefier, the compressed working vapor, the temperature level of which has been raised through the compression, is brought into contact with liquefied working fluid, so that the compressed vapor again liquefies and thus gives off energy to the liquefied working fluid located in the liquefier. This liquefier working fluid is pumped through a heating system by a circulation pump. In particular, a heating flow, at which warmer water is output into a heating cycle, such as a floor heating, is arranged to this end. A heating return then again feeds cooled heating water to the liquefier so as to be heated again by newly condensed working vapor.
This known heat pump may be operated as an open cycle or as a closed cycle. The working medium is water or vapor. In particular, the pressure conditions in the evaporator are such that water having a temperature of 12° C. is evaporated. To this end, the pressure in the evaporator is at about 12 hPa (mbar). By way of the compressor, the pressure of the gas is raised to, e.g., 100 mbar. This corresponds to an evaporation temperature of 45° C. thus prevailing in the liquefier, and particularly in the topmost layer of the liquefied working fluid. This temperature is sufficient for supplying a floor heating.
If higher heating temperatures are required, more compression is adjusted. However, if lower heating temperatures are needed, less compression is adjusted.
Furthermore, the heat pump is based on multi-stage compression. A first flow machine is formed to raise the working vapor to medium pressure. This working vapor at a medium pressure may be guided through a heat exchanger for process water heating so as to then be raised to the pressure needed for the liquefier, such as 100 mbar, e.g. by a last flow machine of a cascade of at least two flow machines. The heat exchanger for process water heating is formed to cool the gas heated (and compressed) by a previous flow machine. Here, the overheating enthalpy is utilized wisely to increase the efficiency of the overall compression process. The cooled gas is then compressed further with one or more downstream compressors or directly supplied to the liquefier. Heat is taken from the compressed water vapor so as to heat process water to higher temperatures than, e.g., 40° C. therewith. However, this does not reduce the overall efficiency of the heat pump, but even increases it, because two successively connected flow machines with gas cooling connected therebetween achieve the demanded gas pressure in the liquefier with a longer life due to the reduced thermal stress and with less energy than if a single flow machine without gas cooling were present.
For a heating system to have acceptance in the market, it should not be too bulky and be offered in a form that can be handled well by workmen or builder-owners can be transported easily to typical locations and be set-up there, such as in cellars or heating rooms.
In particular, in heat pumps operated with water as liquefied working fluid and/or vapor as gaseous working fluid, high demands are made regarding the compressor. In particular, the compressor must have a high output so as to achieve required vapor compression. To this end, it is necessary for a compressor motor to operate at comparably high rotational speeds. Furthermore, in this connection, it is desirable for the compressor to have a radial wheel to achieve efficient, and nonetheless powerful, compression.
High rotational speeds for the motor, however, lead to the fact that the compressor motor contributes to noise development, which may be considerable, especially due to residual imbalances remaining after balancing and/or increasing with the rotational speed of the motor.
Moreover, even very efficient electric motors exhibit heat development also increasing with rising rotational speed, due to the finite ohmic resistances of the current-carrying parts.
The noise development is not that advantageous, indeed, but may be accepted, depending on the setup location of the heat pump, because a heat pump typically is not arranged in the living room, but in a cellar room, which is sonically decoupled from the living room anyway.
Waste heat losses of the motor are, however, even less desirable, because they immediately affect the efficiency of the heat pump. On the other hand, the waste heat requirements may become so high that the motor even has to be cooled actively so as not to loose its specified properties. A special cooling cycle and/or a simple waste heat removal of the motor, e.g. by air convection, however, reduces the efficiency of the heat pump or increases the costs for the heat pump.